The present invention relates to a sheet curl correcting mechanism composed of a rotary object that is either one of an endless annular belt and a roller and of a roller which is in pressure contact with the rotary object, wherein a curled sheet is caused to pass through the contact surface between the rotary object and the roller to be subjected to the bending force in the direction opposite to that of the curl so that the curl of the sheet is corrected, and to an electronic image forming apparatus having therein the curl correcting mechanism.
Next, an example in prior art will be explained referring to drawings. FIG. 4 is a structural diagram of an electrophotographic copying machine representing an example of an electronic image forming apparatus having therein an automatic duplex unit (ADU unit).
In the diagram, the numeral 100 is an electrophotographic copying machine main body, 200 represents a sheet-feeding unit (PFU unit), 300 represents an automatic document feeder (ADC) and 400 represents a finisher device (finisher (sorter with stapler device) FNS device).
The electrophotographic copying machine main body 100 is composed of scanning exposure section 110 which scans an entire surface of document D with a slit-shaped light, then leads optical information reflected on the document D to a photoreceptor drum through mirrors and a lens and forms an electrostatic latent image on the photoreceptor drum, image forming section 120 which visualizes the electrostatic latent image on the photoreceptor drum by the use of toner (developing agent) and transfers it onto copy sheet P, sheet-feeding section 130 which feeds copy sheet P to the image forming section 120, conveyance section 140 which conveys copy sheet P, fixing section 150 which is composed of heating roller 150a and pressure roller 150b and heat-fixes toner on the copy sheet P with heat of the heating roller 150a, sheet-ejection switching section 160, plural sheet-feeding cassettes 170 and automatic duplex unit (ADU unit) 180.
One-dot chain lines in the drawing show a conveyance path for the copy sheet P. The conveyance path includes two types, one is a main route and the other is a circulating route.
The main route is one through which copy sheet P loaded in sheet-feeding cassette 170 or sheet-feeding unit 200 provided in the lower part of the electrophotographic copying machine main body 100 is led to be contained in finisher device 400 through the conveyance section 140, fixing section 150 and sheet-ejection switching section 160 after the image forming conducted by the image forming section 120. The circulation route is a route through which copy sheet P branched from the sheet-ejection switching section 160 is fed again to reach the sheet-feeding section 130 of the electrophotographic copying machine main body 100, after being stocked temporarily in automatic duplex unit 180.
In the electrophotographic copying machine having the aforesaid constitution, the surface of copy sheet P having thereon toner T is heated by heating roller 150a in the fixing section 150 as shown in FIG. 5 (A). In this case, moisture on the surface of the copy sheet on the part of the heating roller 150a and that on the part of the pressure roller 150b differ from each other to cause curl on the copy sheet P.
With regard to the direction of the curl of copy sheet P, W. Gallay's theory has been known, namely, it is known that a sheet is curled with its side dried later than the other side being concaved. Since the surface of copy sheet P having thereon toner T on the part of heating roller 150a is dried first and the surface on the part of pressure roller 150b is dried later, the copy sheet is curled with its surface dried later, namely the surface on the part of pressure roller 150b being concaved as shown in FIG. 5 (B).
Accordingly, when copy sheet P after being fixed is subjected to image forming again through automatic duplex unit 180, a means to correct the curled copy sheet P is needed for preventing a jam caused by poor conveyance.
Curl correcting means in prior art includes the following.
(a) An air flow is applied on copy sheet P which has been fixed to accelerate moisture evaporation on the surface of the copy sheet P on the part of pressure roller 150b and thereby to reduce a difference of moisture from that on the part of the heating roller 150a, so that the curl is corrected. PA1 (b) As shown in FIG. 6, there is a curl correcting means which is composed of driving roller R1, driven roller R2, belt b which is trained about these driving roller R1 and driven roller R2, and roller R3 provided to be in pressure contact with the belt b. In this means, guide G for guiding copy sheet P is provided between the roller R3 and the belt b. PA1 (c) The copy sheet P of this type is conveyed while it is sandwiched by a conveyance mechanism composed of belts and rollers, paired belts, or paired rollers. Conveyance speeds of adjoining conveyance mechanisms which are different from each other apply tension on the copy sheet P in the conveyance direction to correct the curl. PA1 (1) In the sheet curl correcting means in item (a), even when the air flow is applied, moisture on the surface of copy sheet P on the part of pressure roller 150b is not evaporated sufficiently, resulting in insufficient correction of the curl. PA1 (2) An amount of curl of copy sheet P varies depending on surroundings (temperature, humidity etc.) for an apparatus and a type of copy sheet P, and it is not constant. It is therefore necessary to change an amount of curl adjustment depending on environment of the apparatus and the type of copy sheet P in the sheet curl correcting means. PA1 (3) In the case of the sheet curl correcting means as that shown in FIG. 6, a metal subjected to sheet metal processing is used as guide G. PA1 (4) In the case of an electrophotographic copying machine having an arrangement shown in FIG. 4, in the path through which copy sheet P ejected out of fixing section 150 advances to automatic duplex unit 180, there are two portions (portions A and B in FIG. 4) where the radius of curvature is small, and when the copy sheet P passes through these portions, its curl is increased.
When the curled copy sheet P guided by the guide G passes through the contact surface between the belt b and roller R3, bending force in the direction opposite to that of the curl is applied on the copy sheet P, whereby the curl is corrected.
In addition to this, the curl correcting means of this type includes also one which is of a combination of rollers.
However, the curl correcting means in items (a)-(c) above and the electrophotographic copying machine shown in FIG. 4 have the following problems.
In the case of the sheet curl adjusting means as that in item (a) above, it is difficult to adjust the curl because of the constant rate of flow of the air flow.
In the case of the sheet curl adjusting means as that in item (b) above, fine adjustment of the curl is difficult because of dispersion of rubber strength in belts and rollers.
Further, in the case of the sheet curl adjusting means as that in item (b) above, a sheet curl adjusting unit is heated by heat accumulated in copy sheet P heated in the fixing section, causing thermal expansion of rubber on belts and rollers. Therefore, curl correction for the copy sheet P is reduced (or increased) and an amount of adjustment is not stabilized, which is a problem.
The sheet curl adjusting means as that in item (c) above does not employ a method to change an amount of curl adjustment by adjusting a speed difference of the conveyance mechanism.
When the tip of a metallic guide is extended to the vicinity of a pressure contact portion (nip portion) between roller R3 and belt b, it is apprehended that the roller R3 and the belt b are damaged, therefore, curled copy sheet P can not be guided stably, causing a problem that poor conveyance occurs.
In this case, if the radius of curvature at each of the portions A and B is made greater, an increase of curl can be prevented to a certain extent, but there is caused a problem that an apparatus has to be large in size.